Lambert et al., U.S. Pat. No. 3,923,655, describe the desirability of a convenient method of providing oxidation/disinfection on demand for use with potable and recreational water. Those workers note that there are relatively few ways to chemically treat water so that microorganisms are destroyed without leaving behind residual bactericide. The most commonly used treatment is that with chlorine. Other halogens such as bromine and iodine have been used much less frequently, and their usefulness has largely been left to the treatment of swimming pools. Ozone is the only other substance used in large scale treatments.
The Lambert et al. patent teaches the use of strong base anion exchange polymers containing a bactericidally effective amount of triiodide ions (I3−)for killing bacteria, while being said to be essentially free of water-elutable oxidizing iodine. The preferred strong base anion exchange resin used is a quaternary ammonium resin that is first reacted with an alkylating agent to eliminate residual tertiary amine groups.
The need for demand oxidizers/disinfectants; i.e., materials that oxidize and/or disinfect when confronted with a need for such oxidation or disinfection as when contacted with a microbe, is as great now as it was when the Lambert et al. patent was issued. Safe drinking water has become more scarce with increasing population growth, especially in underdeveloped countries where chlorination and other direct chemical treatment or boiling are not viable options. In addition, in more affluent countries, the need for microbial control water for swimming pools, decorative fountains, private wells and ponds has increased dramatically in the years since the issuance of U.S. Pat. No. 3,923,665.
The preferred resin used in the Lambert et al. disclosure is understood to be a co-polymer containing styrene and chloromethylstyrene groups that is cross-linked by divinylbenzene. Those polymerized chloromethylstyrene (vinylbenzyl chloride) groups can be reacted with trimethyl amine to form the quaternary ammonium groups of the strong base anion exchanger. Alternatively, dimethylamine can be reacted with the chloromethyl groups and the tertiary amines so formed can be quaternized by reaction with an alkylating agent such as methyl iodide or dimethyl sulfate.
The co-polymer strong base anion exchange compositions described in the Lambert et al, patent are not widely used in the aforementioned applications because, in practice, they are found to be unstable and bleed objectionable and irritating levels of iodine into the water being treated. This finding is contrary to the express teachings of the patent.
Another problem with the alkylated strong base anion exchange materials described by Lambert et al., and particularly the alkylated quaternary ammonium materials, is that they are themselves not stable, but can decompose to form tertiary amine-containing materials, iodine and methyl iodide. Tertiary amines are poor ligands for triiodide ion and permit that ion to be easily removed. In addition, methyl iodide is listed as being a highly toxic cancer suspect agent in R. J. Lewis, Sax's Dangerous Properties of Industrial Materials, 9th ed., Van Nostrand Reinhold, New York, (1996) page 2262. Lambert et al. teach that one should realkylate a resin prior to forming the triiodide form so that any tertiary amine present would be removed.
Lambert et al. teach that one of the possibly useful resins is an N-alkylated poly(vinylpyridine) as discussed in U.S. Pat. No. 2,739,948. The preparation and use of a similar material is taught in U.S. Pat. No. 5,908,557 for the removal of arsenic anions from aqueous solutions. Those latter co-polymers are known to be incompletely quaternized and to therefore contain some unalkylated, tertiary amine. It is believed that the materials of U.S. Pat. No. 2,739,948 also contained some tertiary amine.
Arsenic poisoning of drinking water has reached catastrophic proportions in some parts of the world. In West Bengal, India, for example, an estimated 200,000 people currently suffer from arsenic-induced skin lesions, some of which have advanced to pre-cancerous hyperkeratoses. In Bangladesh, it is estimated that more than 3 million of the approximately 5 million existing wells are arsenic-contaminated, affecting up to 70 million people--tens of thousands exhibiting symptoms of arsenicosis. The international health community has suggested a target arsenic concentration of no more than 10 parts per billion (ppb) arsenic in drinking water, as compared to the present 50 ppb standard.
In the United States, arsenic in drinking water is designated as a priority contaminant under the 1986 Safe Drinking Water Act and amendments thereto. Since 1974, an arsenic Maximum Contaminant Level (MCL) of 50 ppb has been in effect in the United States. As a result of more recent findings pertaining to health risks associated with populations exposed to high concentrations of arsenic in drinking water, the United States Environmental Protection Agency (EPA) recommends the lowering of the MCL for arsenic from 50 ppb to 2 ppb.
In the United States alone, more than 12,000 public water utilities would fail to meet the more stringent proposed arsenic standard. One estimate places the cost of compliance for the 2 ppb MCL proposal in excess of $5 billion/year. The number of private wells in the United States that fail to meet the existing 50 ppb or proposed 2 ppb MCL for arsenic is unknown. It is believed that in many areas in the USA, many thousands of private wells produce drinking water with potential, serious health risks for the households depending on self-produced water because of arsenic contamination.
Arsenic is found in several oxidation states. Typically, arsenic is present in aqueous solutions in the oxidation state of plus five (As+5, pentavalent) and to a lesser extent the oxidation state of plus three (As+3, trivalent). There is no significant reported cation chemistry for arsenic, but organic arsenic salts are known for both oxidation states (e.g. K[As(C6H4O2)2]).
Examples of trivalent arsenic compounds are the halides (AsCl3, AsCl2+, and AsF3). The halides are readily hydrolyzed to arsenious acid (H3AsO3) or it acid-dissociated forms (HAsO32−). The oxide form is As2O3. The trivalent arsenic compounds to be separated from aqueous solutions, most likely in an ionized form of H3AsO3, in a process of the invention are collectively referred to herein as “trivalent arsenic”.
As0 can be oxidized by concentrated nitric acid to pentavalent arsenic as arsenic acid (isolable as H3AsO4.H2O), which is a moderately strong oxidizing agent in solution. The corresponding halides are also known (e.g. AsCl5, AsCl4+). The pentavalent arsenic compounds to be separated from aqueous solutions are most likely an ionized form of H3AsO4. In a process of the invention, such pentavalent arsenic compounds are collectively referred to herein as “pentavalent arsenic”.
Analytical surveys taken of drinking water around the world usually give a total arsenic level and fail to distinguish contributions from pentavalent arsenic or trivalent arsenic, even though trivalent arsenic is considerably more toxic than pentavalent arsenic. The failure to distinguish the valence of arsenic present in drinking water further confuses the logical assignment of MCL values because although a level of 2 ppb of pentavalent arsenic may cause no deleterious health effects, an equivalent level of trivalent arsenic can have negative health consequences.
There is an urgent need for a technology that will remove arsenic from drinking water to provide safe levels of arsenic regardless of the oxidation state of the arsenic in an efficient, economical and environmentally sound manner. It is desirable that such technology be flexible and sufficiently robust in order to address the requirements of large municipal water utilities, private wells in developed countries and contaminated water sources in undeveloped countries. It is also desirable that an arsenic removal method is able to remove arsenic from water without removing all of the trace minerals that contribute to its flavor.
A number of technologies have been described in the art to remove arsenic from drinking water, also known as “arsenic remediation”. These technologies of the art include iron co-precipitation, reverse osmosis, alumina adsorption and classical ion-exchange with anion exchange resins. These methods can be effective at removing pentavalent arsenic, but trivalent arsenic defies efficient removal. In a report entitled “National Compliance Assessment and Costs for the Regulation of Arsenic in Drinking Water” (January, 1997) prepared by the University of Colorado at Boulder, more than a dozen putative methods are evaluated for arsenic removal efficiency and cost. None of the evaluated methods described exhibited arsenic removal efficiencies greater than 95 percent. In addition, the prior art methods do not offer the simplicity of use required for private well treatment or for less developed areas of the world where reliable electrical power is unavailable. In these situations, a “point of use” treatment is necessary or water must be transported in for use.
U.S. Pat. No. 5,908,557 describes an oxidizing and separation medium for converting trivalent arsenic into pentavalent arsenic, and removing the pentavalent form. That patent relies on the use of polymerized N-alkyl pyridinium triiodide adsorption medium to oxidize and remove the arsenic. Those polymerized N-alkyl pyridinium triiodide moieties can be depicted as shown below, wherein the parentheses are used to show that the polymerized N-alkyl pyridinium triiodide moieties are distributed repeatedly through out the co-polymer thereby forming a plurality of N-alkyl pyridinium triiodide moieties. As is noted at column 5, lines 28-31 of that patent,
not all of the of the pyridine nitrogen atoms are alkylated, thereby leaving some tertiary pyridine nitrogens. More importantly, the presence of N-alkyl groups permits dealkylation of the polymer to occur over time leading to an increased amount of tertiary amine and a methyl halide that can be carcinogenic.
Several other common water contaminants, such as iron(II), antimony, sulfate, nitrate and color-causing contaminants can negatively influence arsenic remediation by the known methods. Classical ion-exchange with anionic resins of the art suffer from poor efficiency (90 percent), low capacity (1500 bed volumes) and severe reduction in capacity and binding efficiency when competing ions such as sulfate are present in amounts of 50 ppm or more. Classical ion-exchange media suffer from poor longevity when challenged with a matrix of hard well water. It is estimated by the aforementioned University of Colorado report that 25 percent of such classical resins would have to be replaced on an annual basis.
The currently-favored plan for removing arsenic from drinking water in Bangladesh is to permit the water to sit for several hours exposed to air, allowing the iron in the water to oxidize, which should cause arsenic to precipitate out and settle.
There remains, therefore, a need for a simple-to-use adsorbent for removing dissolved arsenic from water that is stable and that exhibits an arsenic removal efficiency greater than 95 percent. Another important consideration in water remediation is consumer acceptance. The safety, flavor and cost of the water are all important factors in the provision of drinking water.
It is thus seen that an improved co-polymer triiodide resin is needed to provide demand oxidation/disinfection. Such a material should be stable to decomposition, contain only quaternary ammonium groups while being free of tertiary amine groups, and should neither bleed iodine into surrounding water, nor decompose to form an alkyl halide. The disclosure that follows describes one such co-polymer and several of its uses.